Abstract

Ceramic-on-ceramic articulation shows the lowest wear, with a low incidence of osteolysis. A small cup limits options regarding liner thickness and femoral implant size. The purpose of this study was to evaluate the safety of a 4-mm–thick ceramic liner and the outcomes of cementless total hip arthroplasty using a 44- or 46-mm metal shell and a ceramic-on-ceramic bearing at mid- to long-term follow-up. Between May 2003 and June 2008, 80 patients (88 hips) who had hip osteoarthritis and a hypoplastic acetabulum underwent cementless total hip arthroplasty using a 44- or 46-mm metal shell and a ceramic-on-ceramic bearing. Sixty-eight of these patients (76 hips) were followed for an average of 8 years. The ceramic-related complications, clinical and radiological results, and Kaplan–Meier survivorship of these patients were evaluated. Mean Harris hip score had improved to 88 points (range, 49–100 points) at final follow-up. No ceramic fractures occurred during follow-up. All acetabular and femoral prostheses had bone ingrowth, and there was no measurable wear in any hip. In 1 patient, a radiolucent lesion occurred around the acetabular cup. Two hips underwent revision because of periprosthetic fractures. The survival rates of the acetabular cups and the femoral stems were 100% and 97.4%, respectively. The thin (4 mm) ceramic liner did not increase the risk of ceramic fracture and did not affect the mid- to long-term results and survival of cementless total hip arthroplasty using 44- or 46-mm metal shells. [Orthopedics. 2018; 41(3):e354–e359.]

Abstract

Ceramic-on-ceramic articulation shows the lowest wear, with a low incidence of osteolysis. A small cup limits options regarding liner thickness and femoral implant size. The purpose of this study was to evaluate the safety of a 4-mm–thick ceramic liner and the outcomes of cementless total hip arthroplasty using a 44- or 46-mm metal shell and a ceramic-on-ceramic bearing at mid- to long-term follow-up. Between May 2003 and June 2008, 80 patients (88 hips) who had hip osteoarthritis and a hypoplastic acetabulum underwent cementless total hip arthroplasty using a 44- or 46-mm metal shell and a ceramic-on-ceramic bearing. Sixty-eight of these patients (76 hips) were followed for an average of 8 years. The ceramic-related complications, clinical and radiological results, and Kaplan–Meier survivorship of these patients were evaluated. Mean Harris hip score had improved to 88 points (range, 49–100 points) at final follow-up. No ceramic fractures occurred during follow-up. All acetabular and femoral prostheses had bone ingrowth, and there was no measurable wear in any hip. In 1 patient, a radiolucent lesion occurred around the acetabular cup. Two hips underwent revision because of periprosthetic fractures. The survival rates of the acetabular cups and the femoral stems were 100% and 97.4%, respectively. The thin (4 mm) ceramic liner did not increase the risk of ceramic fracture and did not affect the mid- to long-term results and survival of cementless total hip arthroplasty using 44- or 46-mm metal shells. [Orthopedics. 2018; 41(3):e354–e359.]

Patients who have secondary osteoarthritis due to developmental hip dysplasia or childhood infection frequently have a hypoplastic acetabulum. For these patients, a small (44- or 46-mm) metal shell should be used in total hip arthroplasty (THA). The metal shell should be at least 5 mm thick to avoid deformation during press-fit.1 The minimum diameter of the ceramic head is 28 mm. The thickness of the currently available ceramic liner that matches the 44- or 46-mm metal shell is 4 mm. This thin ceramic liner may be more susceptible to fracture. Although the ceramic-on-ceramic bearing has the lowest wear and high survival at long term,2–4 most manufacturers do not offer a ceramic liner for 44- or 46-mm metal shells. The authors are aware of only one company—Aesculap, Tuttlingen, Germany—that offers 44- or 46-mm metal shells with a 28-mm ceramic head on a 4-mm ceramic liner bearing. However, the safety of this ceramic bearing has not yet been tested.

The purpose of this study was to evaluate the safety and the mid- to long-term outcomes of cementless THA using 44- or 46-mm acetabular cups with an alumina ceramic-on-ceramic articulation.

Materials and Methods

This retrospective study received institutional review board approval, which waived informed consent of the patients. Between May 2003 and June 2008, 88 THAs (80 patients) were performed using small acetabular cups (outer diameters of 44 and 46 mm) and alumina ceramic-on-ceramic articulation at the authors' hospital.

Nine patients (9 hips) were lost to follow-up, and 3 patients (3 hips) died within 5 years postoperatively. None of these 12 patients had ceramic fracture or reported squeaking until last follow-up. The remaining 68 patients (76 hips) were followed for an average of 8 years (range, 5–12 years) (Figure 1). There were 11 men and 57 women. At the index operation, mean age was 48.5 years (range, 18–74 years) and mean body mass index was 24.2 kg/m2 (range, 16–31 kg/m2). The primary diagnosis was dysplasia in 22 hips, septic hip sequelae in 20 hips, osteonecrosis in 26 hips, sequelae of Legg–Calve–Perthes disease in 4 hips, posttraumatic arthritis in 3 hips, and rheumatoid arthritis in 1 hip (Table 1). Mean preoperative Harris hip score was 37 points (range, 15–83 points).

Figure 1:

Flow chart of this study. Abbreviation: THA, total hip arthroplasty.

Table 1:

Baseline Demographics of the Patients by Sex

The authors used the same cementless acetabular cup and cementless femoral stem for all THAs. The acetabular cup was a hemispherical titanium cup (Plasmacup SC; Aesculap) with an outer pure titanium plasma-sprayed coating (Plasmapore; Aesculap). The stem was slightly tapered, rectangular, collarless, and titanium (BiCONTACT; Aesculap). The proximal one-third of the stem was coated with Plasmapore. The neck-shaft angle of the stem was 135°. The standard offset was 40 mm, and the offset of the narrow stem ranged from 31.5 to 35 mm. A stem with the standard offset was used in 64 hips, and a narrow stem was used in 12 hips.

The acetabular insert was an alumina liner (BIOLOX forte; CeramTec AG, Plochingen, Germany). The liner had a truncated shape with a taper angle of 18° at the metal shell–ceramic liner junction. At the equatorial plane of the liner, the outer diameter was 36.825 mm and the inner diameter was 28.0 mm. The thickness of the liner was difficult to define. However, the thinnest width was 4 mm and the thickest width was 5 mm (Figure 2).

Figure 2:

Image showing the thickness of the ceramic liner of the acetabular cup of 44- or 46-mm diameter.

The prosthetic femoral head was a 28-mm alumina ceramic head (BIOLOX forte). A short neck component was used in 48 hips, a medium one in 22 hips, and a long one in 6 hips.

The surgeries were performed by 2 high-volume surgeons (Y.-C.H., K.-H.K.). A Kocher–Langenbeck posterolateral approach was used in 13 hips, a modified anterolateral approach of Mulliken et al5 in 66 hips, and a combined anterolateral and posterior approach in 9 hips. All of the acetabular and femoral components were inserted in a press-fit manner. In hips operated on via the posterior approach, the posterior capsules and short external rotators were tightly repaired to 3 drill holes in the trochanteric crest after implantation of the prosthesis.6

Patients were instructed to walk with partial weight bearing with the aid of crutches for 4 weeks postoperatively. Routine follow-up visits were scheduled for 6 weeks and 3, 6, 9, and 12 months postoperatively and then every 6 months. Patients who did not return for regularly scheduled visits were contacted by telephone. Two nurses and a clinical researcher found and visited nonresponders.

Clinical evaluation was performed using the Harris hip score.7 The radiographic evaluation was performed by 2 independent observers (C.H.P., Y.-K.L.) who did not participate in the index arthroplasty. The 6-week anteroposterior and cross-table lateral radiographs were considered to be the baseline for all comparisons. The abduction and anteversion of the acetabular component were measured on the 6-week radiographs. The method described by Widmer8 was used for the measurement of anteversion. Radiographic evaluation included an assessment of the femoral and acetabular component fixation, ceramic component wear, periprosthetic osteolysis, periprosthetic fracture, and heterotopic ossification. Acetabular components were considered to be loose if there was a change in alignment of greater than 4° or a change in position of 4 mm or greater.9 Femoral components were considered to be unstable when there was progressive subsidence exceeding 3 mm, any change in position, a continuous radiolucent line wider than 2 mm, widening of the femoral canal, or a large pedestal.10 Because there is no method for measuring the wear of ceramic-on-ceramic bearings, the authors used the method for measuring the wear of polyethylene liners developed by Livermore et al.11 Osteolytic lesions were defined according to the criteria of Engh et al.12 The lesions were recorded according to the 3 zones described by DeLee and Charnley13 on the acetabular side and the 7 zones described by Gruen et al14 on the femoral side. Heterotopic ossification was classified according to the system proposed by Brooker et al.15 Kaplan–Meier survival analysis was performed for hips with minimum 5-year follow-up, with any revision of either component as an endpoint.16

Results

A femoral crack occurred in 5 hips during insertion of the stem. These 5 fractures were fixed using cerclage wires and healed completely. Spot welds were seen around all 5 stems, and none of them had migration. Thus, bone ingrowth of the stem was achieved in these 5 hips.

The average anteversion and abduction of the acetabular component were 25.1° (range, 3.7°–49.8°) and 40.16° (range, 26.7°–53.4°), respectively. Two hips dislocated after a stumble at 6 days and 2 weeks postoperatively, respectively. These patients were treated with closed reduction and abduction bracing for 2 months, after which there was no recurrence.

No ceramic fracture occurred and no chipping of the ceramic liner was detected during follow-up. There was no measurable wear in all 76 hips.

One patient had a radiolucent lesion in DeLee and Charnley zones I and II. This was a woman who had undergone THA at age 57 years because of sequelae of Legg–Calve–Perthes disease. During the arthroplasty, the authors inserted 2 screws to obtain rigid fixation of the metal shell because her acetabulum was osteoporotic. The lesion was detected 2 years after the arthroplasty. She is receiving close follow-up, with radiographs every 6 months (Figure 3). At the last follow-up, it could not be determined whether this lesion was osteolytic or stress shielding.

Figure 3:

A 57-year-old woman underwent a total hip arthroplasty because of sequelae of Legg–Calve–Perthes disease (A). Anteroposterior radiograph obtained 6 weeks postoperatively showing a radiolucent area of the acetabular cup in DeLee and Charnley zones I and II (B). Anteroposterior radiographs obtained 2 years (C) and 10 years (D) postoperatively. The radiolucent lesion had not progressed.

All of the acetabular cups and femoral stems had radiographic evidence of bone-ingrown stability at last follow-up.

All femora had cortical thinning, presumed to be due to cancellation of the cortex, limited to the calcar femorale (Gruen zone 7). Thinning was evident at 3 months postoperatively but did not progress on serial radiographs (Figure 4).

Figure 4:

A 40-year-old woman underwent a total hip arthroplasty because of secondary osteoarthritis (A). Anteroposterior radiograph obtained 6 weeks postoperatively showing satisfactory stem fixation and that the acetabular cup was maintained (B). On an anteroposterior radiograph obtained 8 years postoperatively, the stem is fixed by bone ingrowth and cortical thinning of the calcar femorale is observed. The ceramic head is still centric in the acetabulum as a result of no wear of ceramic-on-ceramic articulation (C).

Five femora (6.6%) had cortical hypertrophy in Gruen zone 3 or zone 5 during follow-up. The cortical hypertrophy normalized in 2 hips but persisted until latest follow-up in 3 hips.

Heterotopic ossification developed in 2 (2.6%) hips by last follow-up. Two hips had grade I ossification. One patient (2 hips; 2.6%) had symptomatic deep venous thrombosis. Two femoral stems were revised because of periprosthetic femoral fracture at 5 and 8 years postoperatively, respectively. Retrieved femoral head specimens showed dark stains on the surface, and corresponding ceramic liners also had stains. Those stains looked like metallic smear or stripe wear. However, the authors did not measure surface wear or roughness. No acetabular components were revised until latest follow-up.

Eight patients (8 hips; 9.1%) experienced an intermittent grinding sensation at the operated on hip when rising from sitting or squatting. This phenomenon appeared at 4 to 10 years postoperatively and persisted until final follow-up. However, it was not audible to others. Otherwise, no audible squeaking was noted in any patient until latest follow-up.

Six patients (6 hips; 7.9%) had thigh pain. Two of them had persistent thigh pain at final follow-up. Mean Harris hip score had improved to 88 points (range, 49–100 points) at final follow-up. Of the 76 hips, 55 had excellent, 16 had good, 4 had fair, and 1 had poor results.

Kaplan–Meier survivorship, with any implant revision as the endpoint, was 97.4% (95% confidence interval, 96.7%–100%) at 11.8 years (Figure 5).

Figure 5:

Kaplan–Meier survivorship revealed a cumulative survival rate of 97.4% at 11.8-year follow-up (straight line), with any revision as the endpoint. With an acetabular cup revision as the endpoint, the survival rate would be 100%.

Discussion

Total hip arthroplasty is a successful treatment for patients with end-stage osteoarthritis of the hip. For patients who have a small acetabulum due to hip dysplasia, childhood infection, or a small physique, a small metal shell is required for cementless THA.17 In the current study, the authors defined metal shells with a diameter of 46 mm or less as small cups, as most manufacturers do not produce ceramic-on-ceramic bearings for these metal shells because of concern over fracture of the thin ceramic liner and these criteria have been previously adopted.18,19

Several studies have reported mid- to long-term results of THA using small cups and metal-on-polyethylene bearings. In these studies, the small-diameter cups appeared to perform as well as the larger cups. However, excessive wear and early failure are concerns with thin polyethylene components.17,20,21 Polyethylene liners should have a minimum thickness of 5 mm to avoid early failure related to excessive wear, liner perforation, and fracture.22,23 In small cups less than 46 mm for 28-mm heads and less than 50 mm for 32-mm heads, the liner should be less than 5 mm thick.

In 2014, Maezawa et al24 evaluated 55 metal-on-metal THAs using 44- to 48-mm Wagner stems (Zimmer Biomet, Warsaw, Indiana) standard cups in 55 dysplastic hips. In their study, 17 (31%) acetabular cups showed aseptic loosening during a mean of 3.6 years after surgery. The authors concluded that insufficient osteointegration of the small metal shell design was the main reason for the failure (Table 2). Since the 1990s, contemporary ceramic-on-ceramic bearings have been popular for THA because they have the lowest wear rate among the various articulations. Theoretically, thin ceramic liners are more vulnerable to postoperative fracture. As a result, most manufacturers have not offered and most surgeons have not used ceramic-on-ceramic bearings for THA using small metal shells.

Table 2:

Comparison of Study Results for Total Hip Arthroplasty Using a Small Cup

This is the first study reporting on the outcomes of ceramic-on-ceramic THA using small-diameter (44 or 46 mm) cups and thin (4 mm) ceramic liners. At 5- to 12-year follow-up, there were no ceramic fractures. Although 1 hip had a radiolucent lesion around the metal shell, no hip had detectable wear or prosthetic loosening.

In this study, 6 patients (6 hips; 7.9%) had thigh pain and all patients showed cortical thinning in Gruen zone 7. The authors used one type of cementless stem, which was slightly tapered and long. In a previous study that used the same stem, the incidence of thigh pain was 11.3%.25 The high incidence of thigh pain and cortical thinning may be related to stem design.26

The current study had several limitations. First, 57 (84%) of the patients were women with a low body mass index. Results might differ for male patients and those with a high body mass index.27 Second, one type of metal shell and one type of ceramic liner were used, which had a taper angle of 18°. Results might not be generalizable to other metal shell and ceramic liner designs. Third, BIOLOX forte alumina ceramic was used. Recently, zirconia-reinforced alumina (BIOLOX Delta; CeramTec AG), which is known to have better mechanical properties than alumina ceramic, has replaced alumina ceramic. This newest ceramic can be applied in 42-mm cups with 28-mm or even 32-mm heads of several manufacturers. The authors expect Delta ceramic to perform as well as or better than alumina ceramic in small cups.28 Fourth, wear was measured according to the method of Livermore et al,11 which was developed to measure the wear of polyethylene liners. This measurement cannot detect stripe wear or microwear of ceramic bearings. Fifth, postoperative computed tomography scans were not obtained. Although 2 high-volume surgeons independently and carefully evaluated the radiographs, they may have missed chipping fragments undetectable radiologically.

Conclusion

This study showed that cementless THA using a 4-mm ceramic liner and a 44- or 46-mm metal shell performs well, without ceramic fracture at mid- to long-term follow-up. The authors recommend using ceramic bearings and 44- or 46-mm metal shells for THA in patients who have a small acetabulum.

Image showing the thickness of the ceramic liner of the acetabular cup of 44- or 46-mm diameter.

Figure 3:

A 57-year-old woman underwent a total hip arthroplasty because of sequelae of Legg–Calve–Perthes disease (A). Anteroposterior radiograph obtained 6 weeks postoperatively showing a radiolucent area of the acetabular cup in DeLee and Charnley zones I and II (B). Anteroposterior radiographs obtained 2 years (C) and 10 years (D) postoperatively. The radiolucent lesion had not progressed.

Figure 4:

A 40-year-old woman underwent a total hip arthroplasty because of secondary osteoarthritis (A). Anteroposterior radiograph obtained 6 weeks postoperatively showing satisfactory stem fixation and that the acetabular cup was maintained (B). On an anteroposterior radiograph obtained 8 years postoperatively, the stem is fixed by bone ingrowth and cortical thinning of the calcar femorale is observed. The ceramic head is still centric in the acetabulum as a result of no wear of ceramic-on-ceramic articulation (C).

Figure 5:

Kaplan–Meier survivorship revealed a cumulative survival rate of 97.4% at 11.8-year follow-up (straight line), with any revision as the endpoint. With an acetabular cup revision as the endpoint, the survival rate would be 100%.

Table 2:

Comparison of Study Results for Total Hip Arthroplasty Using a Small Cup

Authors

The authors are from the Department of Orthopaedic Surgery (CHP), Yeungnam University Medical Center, Daegu; the Department of Orthopaedic Surgery (Y-CH), Chung-Ang University College of Medicine, Seoul; and the Department of Orthopaedic Surgery (Y-KL, K-HK), Seoul National University Bundang Hospital, Seongnam, South Korea.

Drs Park and Ha contributed equally to this work and should be considered as equal first authors.